Figure Caption: The Pluto system as we understand it today, with Charon
orbiting in the innermost position, and the two new satellites orbiting
somewhat farther out. Positions and orbits are shown as seen from Earth
for 15 May 2005.

How and when were the new satellites found?

Both satellites were detected using the images obtained by Hubble Space Telescope’s (HST’s) Advanced Camera for Surveys (ACS)
in May, 2005. Our project has its roots in work stretching back to 1990, and our discovery is the result of teamwork
involving all of us. We think the success of this project illustrates both the value of teamwork and the value of persistence.

Although some of us, and others, had been involved in earlier work to search for satellites of Pluto, this team’s quest
to use Hubble to search for satellites of Pluto stretches back almost 3 years.

We began shortly after NASA’s selection of the New Horizons (NH) Pluto mission in November 2002
(see pluto.jhuapl.edu for more information on NH). We began when the NH Principal
Investigator, Alan Stern, initiated an effort to win Hubble time to search for small satellites that may be lurking
undiscovered in the Pluto-Charon system.

NH science team member John Spencer then led an HST proposal submitted in April 2003 for Director’s Discretionary time in
support of NH, but that program was not approved. Despite this, we remained convinced that there might be undiscovered
satellites around Pluto, and we proposed a regular, peer reviewed HST science program in January 2004, this time with Hal
Weaver as the Principal Investigator. This program was also turned down. However, when HST’s STIS spectrograph failed in
early August of 2004, resulting in some valuable HST observing time becoming available, the Space Telescope Science Institute
(STScI) awarded time to a number of highly ranked proposals that has just missed being accepted. As a result, on
September 28th, 2004, we received notification from the STScI that our Pluto companion search program had been
accepted after all!

The Pluto satellite search observations
were scheduled for the spring of 2005 when Pluto was in a good position
for observation. As a result, HST ACS images were made on May 15th and May 18th of 2005. By design, the observations were very sensitive—being
able to detect objects as faint as 27th magnitude in some portions of the images; this is approximately 100,000 times fainter
than Pluto itself, and about half a billion times fainter than the human eye can see.

The images covered enough sky to encompass the entire region that Pluto’s gravity controls—this stretches out over two million
kilometers (1.25 million miles) from Pluto. This was very important to us, because we wanted to completely survey all locations
where satellites of Pluto might lurk.

Our expectation going in to the project
was that Pluto might have one or more faint, distant satellites that
orbit very far from it, as some Kuiper Belt Objects (KBOs) do. No
objects like this were found. However, to our surprise and joy, two
faint bodies fairly close to Pluto were detected.

Max Mutchler first spotted the two satellite candidates, on June 15th,
during an examination of the HST images requested by Principal
Investigator Hal Weaver. He then discussed the possible find with Hal
Weaver, but the potential find needed a lot of follow up work, but they
agreed to defer the necessary follow-up work because both of them were
committed to supporting the ongoing Hubble observations of Comet
9P/Tempel 1, the target of NASA’s Deep Impact mission. Their work on
the Hubble observations of the Deep Impact event kept Hal and Max
completely occupied through the second week of July.

In early July, Alan Stern directed his postdoc Andrew Steffl to sift carefully through all of the HST images in order to see
whether or not Pluto has any faint satellites. Hal and Alan discussed this, and Hal agreed to familiarize Andrew with the data
and to acquaint Andrew with a detailed search strategy during a trip Andrew would make to visit Hal in early August. So they
could perform a completely independent evaluation of the images, Hal did not tell Alan or Andrew of what Max and he thought
they might have found.

After a couple of weeks of work, in mid-August, Steffl reported to Stern on August 18th that he thought he’d found two objects
the day prior. After a few days of follow up, Stern and Steffl became convinced that the two candidate satellites were very
possibly the real deal. Among the follow up tests, we performed were quick look calculations showing that the odds of the
possible satellites being Kuiper Belt Objects or image artifacts were low.

On August 24th Stern contacted Weaver about the potential satellites finds. In that phone call, Hal told Alan that
Max Mutchler and he had found two satellites themselves back in June. Within an afternoon, it became clear that both groups
had found the same objects in the same images.

Weaver and Stern then put together a
Director’s Discretionary (DD) time proposal to use HST to immediately
confirm the potential discoveries. The proposal was submitted to HST on
August 30th. Unfortunately, HST had just entered “2-gyro” observing mode on
August 29th (which had been done to extend HST’s useful lifetime). In 2-gyro mode, HST is limited as to what parts of the sky
it can look at, at any given time. As luck would have it, observations of Pluto would not be technically feasible until
mid-February 2006, so our DD proposal was rejected the next day.

At this point, the team embarked on two parallel paths: one was to more carefully scrutinize the HST data to ensure that no
camera artifacts (e.g., optical ghosts, bad pixels, etc.) were fooling us, and the other was to seek confirming data from any
other (i.e., non-HST) available resource.

Another issue that had to be addressed was whether the objects, even if real, could be background or foreground KBOs not
associated with Pluto. Our calculations showed that the odds of even a single KBO of the same brightness as the candidate
satellites being anywhere in our HST ACS images is <0.1%, and the probability of any given KBO being so close to Pluto is
far smaller still: less than 1 in 100,000. The odds of two KBOs lying so near Pluto in a single image are about 1 in 1 billion.

In September, with the great assistance
of Bill Merline, we attempted to instead confirm the discovery using
some of the largest, best equipped groundbased observatories in the
world: the Keck and Gemini telescopes in Hawaii, and the ESO-VLT
telescope in Chile. However, despite the best efforts of all three
observatories and their staff, the groundbased efforts were not able to
reach faint enough magnitudes to be able to detect the satellite
candidates we found. The problem wasn’t that these telescopes can’t
detect the new satellites—we are convinced they eventually can—it was
just that by September, Pluto was barely visible before setting each
night after twilight.

Merline also discovered that these
objects must be real by demonstrating that the new moons are moving in
orbits that seem to be in the same plane as Charon, and with low
orbital eccentricity—like Charon.

Figure Caption: Although a full orbital solution for the satellites
cannot be determined from only two Hubble measurements, their paths
closely follow that expected for objects orbiting Pluto (or, more
correctly, the Pluto system's barycenter) in a perfect circle in the
same plane as Charon's orbit. In this diagram, the barycenter (i.e.,
the gravitational balance point of the system) is the dot in the
center, Pluto's orbit around it is the smallest ellipse, Charon's orbit
is the next ellipse outward (its positions on 2005 May 15 and May 18
are indicated by the filled and open circles, respectively), an orbit
that is consistent with P2's measured positions is next, followed by an
orbit that is consistent with P1's measured positions. For both the
orbit of P1 and P2, the filled squares are the detected positions of P1
and P2 on May 15 and open squares are the detected positions of P1 and
P2 on May 18. Note that circular orbits look elliptical when they are
projected onto the plane of the sky to mimic what we would see from
HST. The result shown here means that the orbits of P1 and P2 are
likely to be essentially circular, and in the plane of Pluto’s equator,
where Charon’s orbits.

Hard confirmation of P1 and P2 came just last week, on October 24th, when Marc Buie and Eliot Young found the two objects in
several images of an HST dataset they and their collaborators had taken for a Pluto mapping project in 2002.

Figure Caption: A careful search of
HST images taken with the ACS/HRC on June 14, 2002, reveals two objects
that are consistent with the expected locations of the newly-discovered
satellites. One image (left) was taken in yellow light (555 nm) and the
other (right) was taken in blue light (475 nm). The ellipse shows the
orbital path of the new satellites derived from the May 2005 Hubble
observations. Based on the 2005 HST detection dataset, satellites P1
and P2 should lie somewhere along this ellipse. And indeed, there are
two objects along the predicted path (near the red dots), confirming
the detections made with the 2005 HST dataset.

In related news, just 10 days ago, Space Telescope Science Institute director Dr. Matt Mountain has graciously granted us 2
additional HST orbits to re-image the Pluto system, reconfirm the satellites, and improve our knowledge of their orbits. These
observations must wait until February of next year, when Pluto’s position is farther from the glare of the Sun.

What led us to believe the objects near Pluto in the HST images are satellites of Pluto?

Several factors:

First, both satellites appear to be
moving through space with Pluto, and they also appear to be moving
around Pluto, as one can see from the images. Since we commanded the
Hubble Space Telescope to track Pluto during the imaging, objects not
moving with Pluto (like stars and asteroids) appear as streaks in the
images, rather than a point-like source moving with Pluto. It is highly
improbable that an object would appear to be moving with Pluto unless
it was really in orbit around it.

Second, both objects appear to be true
point sources in the images, which is evidence that they are real,
physical objects in space as opposed to optical glints, stray
reflections, or other instrumental signatures in the instrument. HST
ACS camera experts like George Hartig who examined our images do not
believe that any known instrumental effects could generate signatures
that mimic the satellite candidates we had found

Third, the information we have about the orbits of the satellite candidates is consistent with their orbits being in the same
plane as Charon’s, and also nearly circular. This is extremely important, because it is very highly unlikely that any image
artifact or other astronomical body would mimic such motion while also appearing to travel with Pluto.

Fourth, we determined that the new
objects are small enough that they would not have been detectable in
previous (less sensitive) Pluto satellite searches quoted in the
literature, and that their gravitational effects on Charon’s orbit
would not have previously indicated their presence.

Finally, and perhaps most importantly,
Marc Buie and Eliot Young located faint images of both satellites in
HST ACS data taken for a Pluto mapping project they spearheaded in
2002.

How are the two new satellites of Pluto currently designated?

Both satellites have official, provisional designations from the
Central Bureau for Astronomical Telegrams of the International
Astronomical Union (IAU). The brighter satellite is dubbed S/2005 P 1;
the fainter one is dubbed S/2005 P 2. They will receive names at a
later date.

What are the properties of the two new Plutonian satellites?

Very little is known about these bodies at this time. We know their brightnesses, but not their rotation periods, their colors,
their surface reflectivities, or their surface compositions.

Both satellites appear to stray no
farther than 3 arcseconds from Pluto; for reference, Charon orbits no
farther than 0.9 arcseconds from Pluto.

By comparing the positions of the
satellites in different images on different dates, we find that the
positions are consistent with both S/2005 P 1 and S/2005 P 2 having
orbits that are nearly circular and which lie in Pluto’s equatorial
plane. This is a preliminary conclusion, but if it holds up, then the
orbital semi-major axes of P1 and P2 are near 64,700 +/- 850 and 49,500
+/- 600 km, respectively. These semi-major axes in turn correspond to
orbital periods of approximately 38.2 +/- 0.8 and 25.5 +/- 0.5 days,
respectively.

The brighter satellite, S/2005 P 1, has
a V-band magnitude of about 23.0. Because we do not yet know its
surface reflectivity, we cannot definitively calculate its exact size.
But by estimating surface reflectivities and using its distances from
the Earth and Sun at the time of the HST observations, we can determine
the range of plausible sizes it might have. Assuming surface
reflectivities ranging from 4% (like the darkest known KBOs), to 15%
(like many KBOs), to 35% (like Charon), this is what we find:

Assumed Reflectivity

Approximate Diameter

04%

160 km

15%

80 km

35%

52 km

S/2005 P 2 is about 25% fainter than
S/2005 P 1, so could be a 10% to 15% smaller than S/2005 P 1, assuming
they have the same surface reflectivity.

Despite the various uncertainties in
what we know about P1 and P2 so far, and regardless of their surface
reflectivities, these bodies are clearly very small compared to both
Pluto (2284 km diameter) and Charon (1192 km diameter). Indeed, it is
unlikely that either object has a mass larger than 3/10,000th of Pluto’s or 3/1000th of Charon’s.

Might Pluto have other satellites?

Possibly, but because we searched the space around Pluto very thoroughly, we are confident that if Pluto
has any other satellites, they must be much fainter, and therefore much smaller, than either S/2005 P 1 or S/2005 P 2.
Our best estimate is that Pluto has no other satellites larger than 20 km in size.

What is the significance of these bodies if they are confirmed to be satellites of Pluto?

To begin, we now know that the Pluto system is in reality a quadruple system.

With not one but three satellites,
Pluto has become the first body in the Kuiper Belt known to have more
than one satellite. This indicates that many other bodies in the Kuiper
Belt may be multiple-satellite systems.

The discovery will also provide new
probes of the Pluto system. Examples of the information these
satellites will soon yield include improved estimates of the masses and
densities of Pluto and Charon, the possibility of a first estimate of
the J2’s of Pluto and/or Charon, new constraints on the tidal evolution
and origin of the system. When their colors, rotation periods, and
sizes are better known, they will shed more light on their own nature.
And of course, a better understanding of the Pluto system will by
example shed light on the nature and context of other KBO satellite
systems.

Moreover, our discovery means that
NASA’s New Horizons spacecraft will be even busier than originally
planned when it arrives at the Pluto system, since it will have not
two, but four bodies to explore!

What formal scientific publications are planned to report these discoveries?

Our first formal scientific communication consisted of a bulletin released on 31 October 2005 by the International Astronomical
Union (IAU). This bulletin, called an IAU Circular can be found at this link. We have written drafts of three scientific papers that
we plan to soon submit to refereed journals.

These papers can be summarized as follows:

Weaver at al. (2005) reporting the discoveries of P1 and P2 and information on their orbits, brightnesses, and sizes.

Stern et al. (2005) reporting various aspects of interpretation regarding what these discoveries tell us about the Pluto system
in particular and KBO satellite systems in general.

Acknowledgements

We as a team want to thank the people
who built and the people who operate the Hubble Space Telescope. It is
an amazingly capable tool for astronomy. We thank NASA for supporting
the space sciences in many ways, including the funding that keeps HST
operating and the funding that has built New Horizons and made it ready
for launch to the Pluto system. We thank Matt Mountain and his staff at
the Space Telescope Science Institute for working with us to confirm
the discoveries of S/2005 P 1 and S/2005 P 2, and for granting
Director’s Discretionary time for follow up detection observations this
coming February. We thank the directors and observing personnel of the
Keck, Gemini, and VLT observatories for responding to our requests for
observing time on short notice, and for working valiantly to detect P1
and P2 under technically difficult circumstances this fall. We thank
colleagues Keith Noll and Joel Parker for useful conversations as we
worked to confirm P1 and P2, and we thank Jennifer Wiseman, Colleen
Hartman, Melissa McGrath, and Andy Dantzler, all of whom at NASA
Headquarters, for their support of our high standards for confirmation
before releasing news. Finally, we thank the Public Affairs Offices of
NASA Headquarters and Space Telescope Science Institute for their
assistance in preparing the news release and accompanying materials
describing our discoveries of Pluto’s second and third moons.

Caption: Pluto and Charon to scale with each other
and the United States; the new satellites, S/2005 P 1 and S/2005 P 2 are
about half as large as the letter that spells "P" in "Pluto." As
Charon orbits Pluto, it is actually about 19,400 kilometers (about 12,000
miles) away.